1. Field of the Invention
The present invention relates to proton exchange membrane, and in particular relates to a method and composition to improve size stability of the membrane.
2. Description of the Related Art
Nafion® (sulfonated tetrafluoroethylene copolymer) is a conductive polymer developed by Dupont in 1960's. This compound is also called ionomer due to its ionic property that comes from the sulfonatic acid grafted on terminal of the polytetrafluoroethylene bone chain. Nafion has excellent thermal stability and mechanical property, thereby being important in the proton exchange membrane fuel cell (PEMFC) field.
The proton of the sulfonatic acid in the Nafion can transfer from one solfonic acid to another to complete proton exchange. On the contrary, the electron and cation cannot process above transfer.
Although Nafion has many advantages, its size enormously changes after being heated. U.S. Pat. No. 4,983,690 discloses a blend of Nafion and bismaleimide to improve physical properties of Nafion film, however, it still fails to solve the enormous size changes after being heated.
Accordingly, a novel method for improving physical properties of Nafion film is called for.